Such an electrochemical gas sensor has become known from British Patent Application No. GB-2 075 197 A.
The prior-art sensor has a housing pot, in the bottom of which the counterelectrode is accommodated and is in electrical contact with the housing pot, which is also filled with the electrolyte necessary for detecting the gas. The open edge of the housing pot is provided with a circumferential groove, pressed to the inside, on which an electrically insulating sealing ring with L-shaped cross section is placed. The inwardly projecting contact edge of the sealing ring is used to accommodate, in the order beginning from the electrolyte, first a wick disk, over which the disk-shaped measuring electrode is placed, and which latter is in turn covered with a polyfluoroethylene (PTFE) disk acting as a diffusion path. The closure is formed by a disk-shaped cover with a central hole for the access of the gaseous components to be detected. A metallic contact strip, whose length far exceeds the cross section of the sealing ring, is placed adjacent to the electrolyte-side, reaction-sensitive surface of the measuring electrode. After completion of the disk stack, whose closure is formed by the metallic cover, the edge of the housing pot is pressed around the disk tack, clamping same as a whole against the groove provided. The contact strip between the disk edges and the sealing edge is pressed upward in the direction of the metal cover and is folded over, so that it is brought into clamping connection with the measuring electrode surface, on the one hand, and with the metal cover, on the other hand. The housing pot forms one of the electrical contacts of the counterelectrode, and the cover disk forms the other electrical contact for the measuring electrode for connection to a measuring and evaluating unit. Depending on the composition of the electrolyte and the electrode materials used, the prior-art sensor can be used to detect various oxidizing or reducing gases. There are two different embodiments of the prior-art sensor, namely, a so-called two-electrode design, which has only the counterelectrode, besides the measuring electrode, and a so-called three-electrode design, which additionally has a reference electrode, which is maintained at a constant reference potential in relation to the measuring electrode via a potentiostat.
It was found disadvantageous with the prior-art gas sensor that despite the tight pressing, the contact strip placed around the disk stack releases only a small capillary section, through which the electrolyte can penetrate to the closing disk, thus forming, as it were, a short-circuit section, at which a gas/electrolyte-measuring electrode three-phase boundary, which is brought into contact with the counterelectrode, is formed. As a result, an active surface is formed, which, though being small, does distort the measurement result, and is superimposed to the sensor signal. The smaller the desired sensor current, the greater is this disturbing effect. In light of the current increasing miniaturization not only of the evaluating electronic unit but even of the gas sensors themselves, it is of particular significance for the sensors to operate at the lowest possible measuring current in order to reach a long life even in the case of a miniature sensor with small electrolyte reserve. Any interfering current, however small, exerts a measured value-distorting effect. Another disadvantage is the fact that when the edge of the housing pot is pressed onto the housing cover disk, the sensor stack is pressed into the hollow space of the housing pot, so that the individual disks will more or less bulge out, because they lack a central support. As a result, electrolyte films of different thickness are formed between the individual disks, and especially on the measuring electrode surface, so that different diffusion paths will be formed for the gas to be detected, or even very small air bubbles may be retained in the intermediate spaces. Both properties exert an unfavorable effect on the sensor behavior in terms of response time and long-term stability.